1. Field of the Invention
The present invention relates to an aluminum alloy automobile body reinforcement, for example, a bumper reinforcement or a door guard bar which has, as its bending crush characteristics, high bending strength and high energy absorbability against the bending load generated at a time of a vehicle collision and which can provide high collision safety without requiring an additional reinforcement.
2. Description of the Related Art
As is well known, an automobile body is provided with many automobile body reinforcements such as bumper reinforcements and door guard bars. For example, each of the bumpers attached to a front end portion and a rear end portion of an automobile body is, as widely known, internally provided with a bumper reinforcement (also referred to as a bumper armature) for strengthening the bumper. Such a bumper reinforcement has an approximately rectangular cross-section. It is, as well known, positioned between a bumper and an automobile body such that it approximately horizontally extends in the automobile body width direction. The bumper reinforcement makes up, together with the bumper and stays or crashboxes provided behind the bumper, an energy absorption member for absorbing the impact energy generated at a time of a collision.
A bumper reinforcement is fixedly connected, via support members such as bumper stays having an approximately rectangular hollow cross-section, to body frame members such as front side members or rear side members extending in the body longitudinal direction from behind the bumper face that may be subjected to a collision. When an automobile provided with a bumper reinforcement supported as described above collides with an object, the bumper reinforcement absorbs the resultant impact energy by being crush-deformed in the automobile body longitudinal direction and thereby protects the automobile body. Namely, each bumper reinforcement included in an automobile is required to be capable of, when the automobile collides with an object and a large impact load is applied to it, absorbing the impact load energy by being bent-deformed and/or crush-deformed without being broken and flying apart. Other automobile body reinforcements, for example, door guard bars (door beams) with which automobile doors are internally provided to prevent, when the automobile collides at a side thereof with an object, the door on the same side from being deformed into the automobile interior and to thereby protect passengers are also required to have capability and a support structure basically the same as those described above.
In recent years, automobile body reinforcements are made of high-strength extrusions (having a longitudinally uniform cross-section) of, for example, 5000-, 6000-, or 7000-series aluminum alloy instead of steel which used to be in use. Compared with steel on a same weight basis, aluminum alloy excels in impact energy absorbability. As for productivity, aluminum alloy extrusions with a longitudinally uniform, approximately rectangular hollow cross-section offering high strength and rigidity can be efficiently produced in large quantities. Hence, aluminum alloy extrusions can be suitably used for automobile body reinforcements for absorbing impact energy in case of a vehicle collision.
There are various collision tests to which automobile body reinforcements such as bumper reinforcements are subjected. They include, for example, pole collision tests, barrier collision tests, and offset collision tests. Automobile body reinforcements, for example, bumper reinforcements made of aluminum alloy to be put in use are increasingly required to offer enhanced strength characteristics so as to increase safety against a vehicle collision. Take a collision with a pole (also referred to as a pole collision) which the present invention takes into consideration, for example. When an automobile collides with a pole, the load generated by the collision is approximately horizontally applied concentratedly to a local portion of the bumper reinforcement provided for the bumper subjected to the collision of the automobile. In such a case, depending on the magnitude of the load, the crush strength of the bumper reinforcement may turn out inadequate.
The behavior of an aluminum alloy bumper reinforcement at a time of a pole collision will be described more concretely with reference to FIGS. 7A and 7B showing plan views of an automobile. Referring to FIG. 7A, assume that an automobile body A collides with an object C which may be a fire hydrant, an electric pole, or a gatepost. The load generated by the collision is approximately horizontally applied concentratedly to the portion subjected to the collision of a rear bumper reinforcement 110. The same happens to the front bumper reinforcement if the automobile forwardly collides with a similar object. If the load is larger than bearable by the bumper reinforcement 110, the bumper reinforcement 110 may be horizontally bent at a middle portion thereof as shown in FIG. 7B to damage the automobile body A. This can happen, depending on the magnitude of the load, not only to an aluminum alloy bumper reinforcement having an approximately rectangular, simple hollow cross-section but also to an aluminum alloy bumper reinforcement provided with internal ribs for added strength.
To prevent a bumper reinforcement from bending at a time of a pole collision, it is necessary to increase the crush strength of the bumper reinforcement. Measures which can be taken to increase the crush strength of bumper reinforcements may include: increasing the strength of hollow aluminum alloy extrusions used to make automobile body reinforcements such as bumper reinforcements; increasing the wall thicknesses of hollow aluminum alloy extrusions; and increasing the vertical width of bumper reinforcements. There is, however, a limit in increasing the strength of aluminum alloy extrusions. This is because excessively increasing the strength of an aluminum alloy makes it difficult to extrude the aluminum alloy into desired shapes or bend extrusions of the aluminum alloy as desired. Furthermore, excessively strengthened aluminum alloy extrusions can easily crack, so that the amount of energy absorption by bumper reinforcements made of such aluminum alloy extrusions decrease. Increasing the wall thicknesses of hollow aluminum alloy extrusions used to make bumper reinforcements or increasing the vertical widths of bumper reinforcements makes the bumper reinforcements heavier and decreases the advantage of using lightweight hollow aluminum alloy extrusions, so that there is a limit in doing so, too.
A known measure taken for a bumper reinforcement against a possible pole collision is to reinforce a longitudinal middle portion, which is relatively easily bent by an impact load, of the bumper reinforcement by attaching an additional reinforcement thereto. In fact, there are many cases in which an additional aluminum or steel reinforcement is attached to the surface on the colliding side of a bumper reinforcement. To make a bumper reinforcement strong enough by such a measure, however, causes demerits, too. Namely, attaching such an additional reinforcement to a bumper reinforcement increases the total weight of the bumper reinforcement, the total number of parts required, and hence the total cost of the bumper reinforcement. Taking such a measure, therefore, makes the aluminum alloy bumper reinforcement almost as heavy as a steel bumper reinforcement, so that the advantage of using lightweight aluminum alloy is reduced.
In the above regard, it has been proposed to attach an additional reinforcement made of aluminum alloy, instead of steel, to a bumper reinforcement. In the reinforcement structure disclosed in JP-A No. H06-286536, for example, an additional hollow aluminum alloy reinforcement is fixedly bonded to a longitudinal middle portion of a bumper reinforcement. In the bumper reinforcement structure disclosed in JP-A No. 2001-225763, an additional aluminum alloy reinforcement having an open cross-section is provided on the front side of a bumper reinforcement. Furthermore, in JP-A No. 2004-262300, a bumper reinforcement which is made of a hollow aluminum alloy extrusion having an approximately rectangular cross-section and which can be reinforced using existing bumper space is proposed. According to the proposal, an additional reinforcement shorter than a bumper reinforcement and having an approximately rectangular cross-section is integrally attached, to be approximately parallel with the bumper reinforcement, to an upper or lower portion in a longitudinal middle part of the bumper reinforcement.
A technique has been known in which the front wall of a hollow aluminum alloy extrusion having a middle rib is provided with a concave portion formed in a middle portion thereof not as a measure against a pole collision but as a measure to improve the energy absorption performance and the crush strength at a time of an offset collision of the hollow aluminum alloy extrusion (see JP-A No. 2001-26245 and JP-A No. 2002-225652, for example). The concave portion formed in the middle portion of the front wall of the bumper reinforcement increases the crush strength of the front wall.
When a bumper reinforcement is subjected to a pole collision test performed at a high collision speed according to a new severe standard, the crush strength of the bumper reinforcement turns out inadequate even with an existing type of an additional reinforcement attached to the bumper reinforcement. Namely, the bending strength against the bending load generated at a time of a vehicle collision of the bumper reinforcement is inadequate. Moreover, to attach such an existing type of an additional reinforcement to the bumper reinforcement, it is necessary to secure a special installation space in the bumper interior where space availability is limited. This makes it necessary to change the bumper design or automobile body design. In other words, such an additional reinforcement made of aluminum alloy cannot be used in cases where the design of a bumper or automobile body cannot be changed to secure space required to mount the additional reinforcement.
In view of severer collision tests to be passed based on a new standard, only forming, as mentioned above, a concave portion in a middle portion of the front wall of a bumper reinforcement so as to reduce the width-thickness ratio (wall width/wall thickness) and increase the crush strength of the front wall of the bumper reinforcement cannot adequately increase the crush strength against a pole collision of the bumper reinforcement. Namely, the bending strength against the bending load generated at a time of a vehicle collision of the bumper reinforcement is inadequate.
A bumper reinforcement is required to have not only high crush strength against a vehicle collision but also high energy absorbability. When an automobile collides with an object, the load generated by the collision is applied to a bumper reinforcement used in the automobile. When such a collision occurs, the bumper reinforcement is required to prevent the load from quickly decreasing from a maximum level thereof and absorb as much of the load energy as possible so as to reduce damage to the automobile body. The amount of energy absorbed by the bumper reinforcement at the collision is calculated by integrating the amount of the load applied to deform the bumper reinforcement and the amount of deformation of the bumper reinforcement. Other automobile body reinforcements, for example, door guard bars (door beams) are also required to have the energy absorbability as described above. A door guard bar with which an automobile door is internally provided serves to prevent, when the automobile collides at a side of the door with an object, the door from being deformed into the automobile interior and thereby protect passengers.
Thus, automobile body reinforcements are increasingly required to have, as their bending crush characteristics, high bending strength against the bending load generated at a time of a vehicle collision and high energy absorbability. An object of the present invention is to provide an aluminum alloy automobile body reinforcement, for example, a bumper reinforcement or a door guard bar which, having excellent bending crush characteristics, can provide high collision safety without requiring an additional reinforcement.